After a good deal of gnashing of teeth, I finally managed to get Pulseaudio to Do The Right Thing™ (which is what the thing I want, not what it wants): downmixing Surround 5.1 to Stereo.

The problem was simple: I had a lot of lossless FLAC files containing Surround 5.1 audio (that is, 6 channels). However, at the moment I only own a couple of speakers which are good enough for casual listening (ergo, 2 channels stereo).

I thus wanted Rhythmbox, Totem, and other apps to simply make me listen to the rear channels; else, I would lose half of the guitar solos in Jethro Tull’s “Aqualung”, because when it was recorded it was spatially at the rear of the room.